Process for production of chlorine dioxide

Abstract

A mixture of sulfuric acid and hydrochloric acid essentially free from dissolved sulfur dioxide to be used in a chlorine dioxide reactor is produced by reacting chlorine and sulfur dioxide in at least two reaction zones. In the bottom of a primary reaction zone the total amount of sulfur dioxide needed for the mixed acid production is introduced in an aqueous mixture of sulfuric and hydrochloric acid containing dissolved chlorine and gaseous chlorine. The sulfur dioxide reacts with chlorine in the aqueous phase to produce sulfuric acid and hydrochloric acid. The mixed acid produced in the primary reaction zone contains dissolved quantities of sulfur dioxide and is forwarded to a secondary reaction zone. Chlorine is introduced into the lower part of the secondary reaction zone. The amount introduced can be the total amount of chlorine to be used in the production of the mixed acid for the chlorine dioxide reactor, or it can be a part of that amount, the balance being introduced in the primary reaction zone. The amount should be large enough for a gaseous chlorine phase to leave the secondary reaction zone. The produced mixed acid is withdrawn from the lower part, and preferably the bottom of the secondary reaction zone. At least a part of the mixed acid is introduced into a chlorine dioxide reactor.

We claim: 
     
       1. A process for production of chlorine dioxide which comprises: (a) reacting in a chlorine dioxide generator sodium chlorate with chloride ions and sulfuric acid in an acid normality of about 2 to about 4.8 normal, said chloride ions being provided by hydrochloric acid or by a mixture of hydrochloric acid and sodium chloride, in proportions to generate chlorine dioxide;   (b) maintaining said reaction medium at a temperature from about 50° C. to about 100° C.;   (c) subjecting the reaction solution to a subatmospheric pressure sufficient to effect evaporation of water;   (d) precipitating sodium sulfate from the reaction medium and removing it from the reaction zone;   (e) withdrawing a mixture of chlorine dioxide, chlorine and water vapour and forming an aqueous solution of the chlorine dioxide and a gaseous chlorine stream therefrom;   (f) forwarding said chlorine to a production zone for hydrochloric acid and sulfuric acid comprising a primary reaction zone and a secondary reaction zone;   (g) introducing sulfur dioxide and chlorine into the lower part of the primary reaction zone in contact with a fluid stream which contains hydrochloric acid and sulfuric acid, at least a portion of said chlorine coming from the secondary reaction zone, whereby said sulfur dioxide reacts with the chlorine in said primary reaction zone to form a fluid mixture of hydrochloric acid and sulfuric acid containing residual sulfur dioxide;   (h) forwarding said fluid mixture of acids, which contains residual sulfur dioxide, from said primary reaction zone, to the secondary reaction zone and introducing chlorine, in an amount sufficient for gaseous chlorine to leave said secondary reaction zone and in an amount exceeding that which stoichiometrically corresponds to the reduction of the sulfur dioxide content in the mixed acid, into the lower part of said secondary reaction zone downstream of the sulfur dioxide feed stream in the primary reaction zone and in contact with said fluid mixture, the chlorine reacting with the sulfur dioxide and thereby reducing the sulfur dioxide content in the mixture of acids flowing through said secondary reaction zone, the chlorine and the sulfur dioxide being introduced into the production zone in substantially stoichiometrically equal amounts; and   (i) forwarding at least part of the mixture of acids passing through said secondary reaction zone to the chlorine dioxide generator, and withdrawing gaseous chlorine from the top of the secondary reaction zone and passing it to the lower portion of the primary reaction zone.   
     
     
       2. A process according to claim 1, wherein the primary and secondary reaction zones for the reaction of chlorine and sulfur dioxide are contained in a single vessel. 
     
     
       3. A process according to claim 1, wherein substantially only the chlorine produced in the reaction zone is introduced into the production zone. 
     
     
       4. A process according to claim 1 wherein chlorine in addition to the chlorine coming from the secondary reaction zone is introduced at the bottom of the primary reaction zone. 
     
     
       5. A process according to claim 1, wherein a portion of the mixed acid withdrawn from the secondary reaction zone is forwarded to the chlorine dioxide generator and the remainder is recirculated to the upper part of the primary reaction zone. 
     
     
       6. A process according to claim 1, wherein the primary and secondary reaction zones are contained in separate vessels. 
     
     
       7. A process according to claim 1, wherein at least part of the total chlorine is introduced into the secondary reaction zone in an amount sufficient for gaseous chlorine to leave the secondary reaction zone and in an amount exceeding that which stoichiometrically corresponds to the desired reduction of the sulfur dioxide content of the mixed acid and wherein the gaseous products from the secondary reaction zone are introduced into the lower part of the primary reaction zone together with the remainder of the chlorine. 
     
     
       8. A process according to claim 6 wherein a portion of the mixed acid withdrawn from the primary reaction zone is forwarded to the secondary reaction zone and the remainder is recirculated to the upper part of the primary reaction zone. 
     
     
       9. A process according to claim 7 wherein the mixed acid withdrawn from the secondary reaction zone is forwarded to the chloride dioxide generator. 
     
     
       10. A process for production of chlorine dioxide which comprises: (a) reacting in a chlorine dioxide generator sodium chlorate with chloride ions and sulfuric acid in an acid normality of about 2 to about 4.8 normal, said chloride ions being provided by hydrochloric acid or by a mixture of hydrochloric acid and sodium chloride, in proportions to generate chlorine dioxide;   (b) maintaining said reaction medium at a temperature from about 50° C. to about 100° C.;   (c) subjecting the reaction solution to a subatmospheric pressure sufficient to effect evaporation of water;   (d) precipitating sodium sulfate from the reaction medium and removing it from the reaction zone;   (e) withdrawing a mixture of chlorine dioxide, chlorine and water vapour from the reaction zone and forming an aqueous solution of the chlorine dioxide and a gaseous chlorine stream therefrom;   (f) forwarding said chlorine to a production zone for hydrochloric acid and sulfuric acid comprising a primary reaction zone and a secondary reaction zone;   (g) introducing sulfur dioxide and chlorine into the lower part of the primary reaction zone in contact with a fluid stream containing hydrochloric acid and sulfuric acid, at least a portion of said chlorine coming from the secondary reaction zone and the remainder of said chlorine coming from step (e), said sulfur dioxide and chlorine reacting in said primary reaction zone to form a fluid mixture of hydrochloric acid and sulfuric acid containing residual sulfur dioxide;   (h) forwarding said fluid mixture of acids formed in step (g), and which contain residual sulfur dioxide, to the secondary reaction zone and introducing chlorine, in an amount sufficient for gaseous chlorine to leave said secondary reaction zone and in an amount exceeding that which stoichiometrically corresponds to the reduction of the sulfur dioxide content in the mixed acid, into the lower part of said secondary reaction zone downstream of the sulfur dioxide feed stream in the primary reaction zone and in contact with said fluid mixture, a portion of the thus introduced chlorine reacting with sulfur dioxide to thereby reduce the sulfur dioxide content of the fluid mixture flowing through said secondary reaction zone, the chlorine and the sulfur dioxide being introduced into the production zone in substantially stoichiometrically equal amounts; and   (i) forwarding at least part of the mixture of acids passing through said secondary reaction zone to the chlorine dioxide generator, and withdrawing gaseous chlorine from the top of the secondary reaction zone and passing it to the lower portion of the primary reaction zone.